fix: Match TextBox.VerticalContentAlignment with Windows

[Youssef1313]

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GitHub Issue (If applicable): closes #4502.

This is a revert of https://github.com/unoplatform/uno/pull/4180

PR Type

What kind of change does this PR introduce?

What is the current behavior?

What is the new behavior?

Copilot Summary

🤖 Generated by Copilot at 5dd22d4

This pull request fixes a layout bug in TextBox controls and adds a unit test to verify the fix. It also improves the XAML loading logic by using .NET 7.0 source generators and a helper method LoadXaml.

PR Checklist

Please check if your PR fulfills the following requirements:

• [ ] Docs have been added/updated which fit documentation template (for bug fixes / features)
• [ ] Unit Tests and/or UI Tests for the changes have been added (for bug fixes / features) (if applicable)
• [ ] Validated PR Screenshots Compare Test Run results.
• [ ] Contains NO breaking changes
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Internal Issue (If applicable):

[Youssef1313]

/azp run

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[Youssef1313]

There is a problem on Skia where the native Gtk.Entry (shown only on focus), has the text centered vertically, and Gtk.Entry provides no way to change this behavior.

So:

• Existing behavior on Skia: On both focused & unfocused state, the text is centered vertically.
• New behavior on Skia: On focus, it's centered vertically. On unfocus, the text is top-aligned.
• Windows behavior: Text is top-aligned.

So while this PR gets closer to Windows behavior on Skia, I think it could be considered worse because of the inconsistency between focused and unfocused states.

[Youssef1313]

I guess @ramezgerges work will probably help unblock this. Let's see :)

[MartinZikmund]

@Youssef1313 I think the new behavior is acceptable - it will work properly with rendered TextBox and it works well with WPF

[Youssef1313]

@MartinZikmund Yup it should work well with the new TextBox implementation, but not the old one which is still the "default":

https://github.com/unoplatform/uno/blob/efa47a5bb0a567e6f39dc44362d430507f744f2d/src/Uno.UI/FeatureConfiguration.cs#L505

[MartinZikmund]

@Youssef1313 but that is the default only for now and will "change" soon, and WPF works fine so I don't see this as a big issue

[MartinZikmund]

@Youssef1313 I think this can be revived now after 5.2

[Youssef1313]

Yup. Sorry forgot about it earlier

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[unodevops]

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[MartinZikmund]

Ready to be merged!